Comparison of a Rebound Tonometer with the Tonopen XL for Measuring IOP in a Ocular Hypertensive Rat Model. Effects of Experience and Anesthesia

Abstract

Abstract: : Purpose: To compare a rebound tonometer (RT) with the Tonopen for measuring IOP in rats and determine the effect of prior experience in the use of the RT and its appropriateness for measuring IOP in anesthetized and awake rats. Methods: The episcleral veins of one eye of Brown-Norway rats were injected with hypertonic saline to induce IOP elevation. IOP was measured in both eyes of awake rats with the Tonopen and the RT at the same time on 3 occasions, 1-17 days after the injection. For these 3 sets of measurements the operators had no prior experience with the RT but were well experienced in use of the Tonopen. 4-6 weeks after the saline injection, 13 of the rats were anesthetized and IOP was again measured but by operators experienced in the use of both tonometers. The 4 sets of IOP measurements with the RT were compared and correlated with those from the Tonopen. In addition ΔIOPs (IOP difference between contralateral eyes) as determined with the 2 tonometers were correlated. Operator experience in RT use was investigated by comparing correlation coefficients among the 3 sets of measurements from awake animals and those from anesthetized animals. Results: Mean (±SD) IOP for all sets of measurements in awake animals with the tonopen was 30.86 (±5.46) mmHg, and with the RT 25.74 (±5.06) mmHg (p<0.0001, paired t-test). The overall correlation of IOPs as well as ΔIOPs by the two instruments was poor (R²=0.01 and 0.09 respectively).However, both correlations improved with repeated use of the RT, reaching values of R²=0.11 for IOPs and R²=0.25 for ΔIOPs by the 3^rd time the tonometers were used. Correlations also improved for both comparisons with increasing time from the saline injection, reaching values of R²=0.42 for IOPs and R²=0.65 for ΔIOPs on day 17. 2-4 weeks later, when the rats were anesthetized, mean IOP for all measurements with the Tonopen was 29.01(±13.73) mmHg while only 21.97 (±15.37) mmHg with the RT(p<0.001, paired t-test). IOPs and ΔIOPs showed excellent correlation between the two tonometers (R²=0.94 and 0.96 respectively). Conclusions:The RT calibrated on cannulated rat eyes ex-vivo gives lower IOP values in vivo than the Tonopen. RT IOP measurements on anesthetized rats performed by experienced operators correlate very well with Tonopen measurements. Correlation is much less for measurements in awake animals by operators without significant prior experience with the RT. However, this correlation improves rapidly with increasing experience in the instrument use.

Keywords: intraocular pressure • animal model